MAINTAINERS: update drm bug reporting URL

The original bugzilla seems to be read-only now, linking to the gitlab
for new bugs.

Signed-off-by: Pavel Turinský <ledoian@kam.mff.cuni.cz>
Cc: trivial@kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210228163658.54962-1-ledoian@kam.mff.cuni.cz
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>

MAINTAINERS

@@ -5835,7 +5835,7 @@ M:	David Airlie <airlied@linux.ie>
 M:	Daniel Vetter <daniel@ffwll.ch>
 L:	dri-devel@lists.freedesktop.org
 S:	Maintained
-B:	https://bugs.freedesktop.org/
+B:	https://gitlab.freedesktop.org/drm
 C:	irc://chat.freenode.net/dri-devel
 T:	git git://anongit.freedesktop.org/drm/drm
 F:	Documentation/devicetree/bindings/display/